Field of the Invention
The present invention concerns a method for recording a magnetic resonance dataset. The invention also relates to a data storage and a magnetic resonance apparatus that implement such a method.
Description of the Prior Art
A problem that can occur in magnetic resonance scans is the occurrence of signal voids as a result of susceptibility jumps in the tissue of the region being examined.
For example, it is known that signal voids occur at water/air transitions. This is caused by the relationship of the magnetic field strength B with the magnetic excitation H and the magnetization J:B=μ0(H+J).
Since the magnetization J can also be expressed asJ=χHthe following is obtainedB=μ0μrH=μ0(1+χ)H=B0+μ0J. 
Therefore, a field change ΔB takes place at the boundary between two tissues or materials with different χ values:ΔB=μ0(χ1−χ2)H 
The additional gradients present at the boundaries result in dephasing of the spins, which results in signal losses.
These signal losses caused by susceptibility jumps in different tissues are also called susceptibility artifacts. These occur, inter alia, in images of the lungs or the head in the region of the nose, the frontal sinus or the auditory canals. This problem also occurs with implants, i.e. with water/metal transitions.
In this case, the degree of the susceptibility jumps depends upon the χ values. The jumps are particularly strong at water-air or water-metal transitions.
In this case, the field change ΔB generates a so-called moment. This is generally defined as follows:M=∫G dt, where G describes the spatial gradient of the field change along a specific axis and dt describes the differential of the time integral.
Several strategies are known for the reduction of susceptibility artifacts:
First: susceptibility artifacts can be eliminated by the use of spin-echo-based recording methods. In this case, one or more 180° refocusing pulses are present.
It is also possible to reduce susceptibility artifacts if the axis of the jump in susceptibility, if there is such an axis, is parallel to B0. This procedure is useful, for example, when positioning surgical needles, see Ladd et al., Biopsy needle susceptibility artifacts, Magn Reson Med, 36(4), pp 646-651, 1996.
It is also possible to reduce susceptibility artifacts—so-called “in-plane dephasing” in the read and/or phase direction—in gradient-echo sequences by reducing the echo time TE. It is known from Port and Pomper, Quantification and Minimization of Magnetic Susceptibility Artifacts on GRE Images., J Comput Assist Tomogr, 24(6), 958-964, 2000 that that this makes the greatest contribution to the minimization of artifacts. With respect to pulmonary imaging, it is known from Jakob et al., Rapid Quantitative LUNG 1H T1-Mapping, J Magn Reson Imaging 14, pp. 795-799, 2001 to record FLASH images with an echo time of 1 ms in order to minimize susceptibility artifacts.
Furthermore, Du et al., Reducing Susceptibility Artifacts in fMRI Using Volume-Selective z-Shim Compensation, Magn. Res. Med., 57, pp. 396-404, 2007 and Schneider et al., Automated Slice-Specific Simultaneous Z-Shim Method for Reducing B1 Inhomogeneity and Susceptibility-Induced Signal Loss with Parallel Transmission at 3T, Magn. Reson. Med., 74, pp 934-944, 2014 describe so-called z-shim correction. In this case, in addition to the artifact-affected image datasets, image datasets are recorded using an additional moment in the slice direction, which corrects so-called “through-plane dephasing” and assembles the image datasets to form a combination image. This procedure is described by the term “z-shim”. However, in this case, the calculation of the additional moments is partially empirical and time-consuming.